THE MODERN RIO BERMEJO MEGAFAN, CHACO FORELAND BASIN (ARGENTINA)

Fluvial megafans are important to the stratigraphy of retroarc foreland basins. Megafan deposits are frequently the dominant architectural elements that build stratigraphic sequences in the foredeep depozone. As a consequence, a robust comprehension of these depositional environments is critical for successful natural resource exploration, as well as elucidating ancient patterns of sedimentation, hydrology, climate, and tectonic processes. The goal of this project is to use actualistic data to further our understanding of fluvial megafans in the Chaco foreland deposystem. The low-gradient Río Bermejo megafan of northern Argentina provides an ideal study site towards this goal.

The geography of the Río Bermejo watershed was analyzed using ArcGIS, in order to assess the potential controls on sediment generation and deposition in the Bermejo megafan. Key components of the GIS database included: watershed boundaries, geology, topography, climate, and vegetation maps. A conditioned digital elevation model was also developed to assess geomorphological features of the megafan surface. Further, a comprehensive suite of modern sediments from across the Río Bermejo watershed underwent a three-pronged analysis to determine composition and provenance characteristics using classic thin section petrography (Gazzi-Dickinson method; n=40), detrital zircon geochronology (n=5), and x-ray diffraction for clay mineralogy (n=26).

Initial results indicate that three distinct upstream “regions” feed sediment and water to the Bermejo megafan: (1) the Ríos San Francisco/Grande region; (2) the Río Bermejo headwater region; and (3) the Santa Barbara massif region. Parent lithologies, slope, type and extent of plant life, precipitation, and discharge characteristics vary markedly across these regions. The composition and age of sands and the mineralogy of detrital clays in the Bermejo megafan reflect these upstream controls. Further analysis will expand our understanding of the modern Chaco foreland basin in general and the sediment budget for the Bermejo megafan in particular.